This invention relates generally to an improved comparator circuit and, more particularly, to improved comparator circuits suitable for use when the input voltage to be compared may be greater than the comparator circuit's supply voltage. More particularly, the invention relates to an improved comparator circuit suitable for use in conjunction with other comparators on a common resistive divider chain.
The most typical comparator circuits are of the type which comprise input NPN transistors, the base electrodes of which may be coupled to the input terminals of the comparator. In this case, the input bias currents (i.e. the base currents of the input NPN transistors) are small. Unfortunately, such comparator circuits have application only when the input voltages to be compared are less than the circuit supply voltage.
In automotive applications, however, it is necessary that a comparator circuit be capable of comparing voltages higher than the supply voltage. Furthermore, it is desirous that several such comparator circuits be coupled to a common resistive divider chain for comparing different voltages on the chain. Due to the relatively high input voltages (compared to the supply voltage), NPN transistors are not suitable as input transistors, and PNP transistors are employed. PNP transistors, however, draw relatively large input bias currents (e.g. 10-20 micro amperes) thus affecting the resistor ratio of the divider chain and reducing the accuracy of the comparators.